As Denver-based Frontier Airlines continues the transition to becoming the nation’s newest ultra-low-cost carrier, the rate of passenger complaints has soared.

In the latest data reported by the U.S. Department of Transportation, Frontier had 123 complaints filed in February, or 14.4 complaints for every 100,000 passenger boardings. In contrast, the top 13 airlines in the country had a combined rate of 1.97 complaints for every 100,000 passenger boardings.

The complaint rate for Frontier has been on the rise. In November, the rate was 3.3 complaints for every 100,000 boardings, before rising to 4.14 in December and then 8.61 in January, according to the Department of Transportation.

The carrier was purchased in 2013 by the same private investors who helped convert Florida-based Spirit Airlines into an ultra-low-cost carrier with dozens of passenger fees. Frontier has already adopted new fees for carry-on bags and other extras.

Frontier acknowledges that “some complaints stem from our transition into an ultra-low-cost carrier as we focus on bringing everyday low fares to more of the country,” Frontier spokesman Todd Lehmacher said in a statement.

He said many complaints were the result of long delays that customers had to endure trying to talk to reservation agents — a problem the carrier hopes to address by hiring more staff for its call center.

But another reason for complaints may be Frontier’s below-par on-time performance rate. In February, Frontier arrived on time an average of 59% of the time, the second-lowest rate for all major airlines in the country.

Lehmacher expects complaint numbers to also be high for March but to drop for April.

“We are focused on improving and believe we have turned a corner,” he said.

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